Stressful and emotional events can promote or impair the acquisition of new memories, depending on the type of stressful experience, the type of learning—even the sex of the animal. Stress is the external condition that places demands on the organism, and the stress response is the organism's adaptive response to the stressor, typically measured as changes in performance or physiological or biochemical states.

Most adaptive responses are crucial to an organism's capacity for survival and are easily reconcilable with theories of natural selection. For instance, the release of glucocorticoids from the adrenal glands directs glucose to the brain and musculature in preparation for "fight or flight," thereby eliminating unnecessary processing of ongoing vegetative functions such as digestion. These physiological responses to stress in turn affect cognitive processes such as learning and memory, thereby allowing the animal to prepare for or avoid subsequent sources...